Only two months ago interest in women's tennis had plummeted to an all-time low, with no apparent challengers to the all-conquering Williams sisters on the horizon.

Everything has changed over the last fortnight here at the US Open. The extraordinary return of Kim Clijsters, who was playing in last night's final in only her third tournament after a two-year break from the game, the emergence of teenagers like Caroline Wozniacki, her opponent, and Melanie Oudin, plus talk of a comeback by Justine Henin, have instantly transformed the women's game.

The Women's Tennis Association would no doubt prefer that the headlines from Flushing Meadows stopped there, but this US Open is likely to be remembered above all for Serena Williams' extraordinary departure from the tournament on Saturday night, when she was penalised on match point to Clijsters in the semi-finals after a foul-mouthed tirade directed at a female line judge.

Serving at 5-6 and 15-30, Williams was foot-faulted on her second serve, upon which she confronted the line judge and threatened to "take this ball and shove it down your throat". Having already received one code violation for smashing her racket at the end of the first set, Williams was given a second and an automatic one-point deduction, handing Clijsters a 6-4, 7-5 victory.

If it was an almost unprecedented decision to foot-fault a player at that stage of such a high-profile match – particularly as television replays showed that Williams' foot had barely touched the line if at all –there could be no excusing the American's behaviour. She will be fined, though officials were still considering her punishment yesterday morning.

The controversy overshadowed the fact that Williams, the defending champion, had been outplayed by Clijsters, who was making her first appearance at a Grand Slam event since the 2007 Australian Open after retiring to start a family. Her comeback in two tournaments last month showed that she would be competitive against the best players, but to have achieved what she has here as a wild card is a remarkable performance.

Clijsters watched from the other end of the court as the extraordinary finale unfolded. Following Williams' outburst, the line judge was called to the umpire's chair while Brian Earley, the tournament referee, came on to the court. Williams joined in the conversation and protested: "Sorry, but there are a lot of people who've said way worse." It was not clear what the line judge had said to the umpire, but Williams told her: "I didn't say I would kill you. Are you serious? I didn't say that."

Earley explained later: "She was called for a foot fault and a point later she said something to a line umpire, and it was reported to the chair and that resulted in a point penalty. It just happened that point penalty was match point. It was a code violation for unsportsmanlike conduct."

After the match Williams was unrepentant. "All year I've never been foot-faulted and then suddenly in this tournament they keep calling foot faults," she said. "I said something that I guess they gave me a point penalty for. Unfortunately it was on match point."

Did Williams agree that she had threatened the line judge? "Well, I've never been in a fight in my whole life, so I don't know why she would have felt threatened," the world No 2 replied.

When asked if the official deserved an apology, Williams replied: "An apology? From me? Well, how many people yell at lines people?" She added: "If you look at all the people that kind of yell at lines people, I think it kind of comes sometimes. Players, athletes get frustrated. I don't know how many times I've seen that happen."

The way the match finished denied Clijsters the chance to celebrate her achievement in reaching the final of a tournament she last contested four years ago, when she won the title here to claim her only Grand Slam.

"It's unfortunate that a match that I was playing so well had to end that way," she said. "Obviously, I am a little confused about what happened, just because I was so focused. Things ended a bit different than I expected."